Conventionally, small and medium sized vehicles are provided with hydraulic brake systems having vacuum assistance via vacuum boosters. Historically the source of vacuum was from the inlet manifold of a gasoline engine, or from a vacuum pump of a diesel engine. More recently, vacuum pumps have been provided for both gasoline and diesel vehicles.
Dry running vacuum pumps driven by an electric motor have been proposed, but, for reliability and long life, an oil-lubricated mechanically driven vacuum pump is often preferred. Such a pump is typically driven directly from an engine camshaft, though other mechanical arrangements are possible.
Engine driven vacuum pumps rotate continuously, and exert a small drag on the vehicle engine, due to friction and pumping losses. It is desirable to minimize such parasitic loss, so as to improve overall fuel consumption of the vehicle, especially since vacuum pumps may not be required for long time periods—for example when driving on highways where brake application is infrequent.
It has been proposed (e.g., in EP2049355A) to provide a disengageable friction clutch whereby drive to the pump rotor can be engaged and disengaged on demand. However, a friction drive may be problematic under cold start conditions (−30° C.), because a high drive torque may be required to clear lubrication oil accumulated in the vacuum pump. Such a high torque may result in reduced clutch life, which is not compatible with failsafe operation. Furthermore, a friction clutch may be physically large.
Equally, a mechanical drive connection, such as a dog clutch is not considered practicable because of the shock loading when the drive to the vacuum pump is connected.
What is required is a failsafe disconnectable drive for a vacuum pump, which is capable of cold start engagement without shock loading, and which preferably does not require electrical components or electrical connections.